1. Field of the Invention
This invention relates to a screw rivet for coupling together a plurality of overlapped panels, or the like.
2. Description of the Prior Art
Prior art screw rivets of this type comprise a screw which includes a head and a threaded pin depending from the lower surface of the head and having external threads, and a grommet which is a one-piece plastic molding and includes a flange facing the head of the screw, a leg depending from the lower surface of the flange, a bore extending through the flange into the leg and permitting insertion of the threaded pin of the screw, ridges provided on the inner surface thereof defining the bore at an intermediate position thereof and each meshed with the groove between the adjacent external threads of the screw, a plurality of radially spaced-apart axial slits splitting the leg, and a plurality of expansible portions individually located between adjacent slits and capable of being pushed and expanded by the threaded pin of the screw inserted into the bore (as disclosed in U.S. Pat. Nos. 4,311,421, 4,878,791, 4,478,545 and 4,367,995, for instance).
When using such a prior art screw rivet for coupling together a plurality of overlapped panels, the leg of the grommet is inserted through holes formed in the panels until the lower surface of the flange comes into contact with the uppermost panel, and then the external threads of the screw are forcibly pushed into the bore of the grommet to cause expansion of the expansible portions of the grommet, thus effecting the coupling of the panels. When removing the screw, it is turned in the direction which causes it to rise owing to the meshing of the thread grooves and the ridges of the grommet.
However, in the prior art screw rivet noted above, the ridges of the grommet project toward the axis of the bore thereof. Therefore, when inserting the screw, the external threads thereof and the ridges of the grommet strike and clear each other, and in this way the external threads of the screw forcibly proceed into the bore of the grommet. At this time, both the external threads and the ridges are squeezed or otherwise deformed or damaged. Consequently, the degree of meshing between the thread grooves and the ridges is reduced, thus adversely affecting the screw turn-up operation.
In molding a grommet, the bore thereof is formed by extracting a core pin. However, since the ridges project inwardly within the bore, the core pin has to be forcibly extracted. If the extent of projection of the ridges is increased, forcible extraction of the core pin becomes difficult. For this reason, the extent of projection of the ridges cannot be increased. Inevitably, therefore, the ridges are meshed with the thread grooves only slightly, leading to an incomplete screw turn-up operation.